Thanks for the responses. There are no signs of any tampering or any other indications where the coins could accidentally slip out, so I will take them to my friendly post office and ask them (nicely) to weigh them for me to double check for any hidden coins. But, I have carefully felt the packaging for dislodged coins and I'm pretty sure they aren't there.
I forgot to specify that I bought the 2004 set from either the RAM website or RAM order form back (can't remember which now) in 2004 or 2005, so there is absolutely no chance of tampering there. At the time I rang the Mint to complain about it and they said it often happens and to send it back for a replacement. I opted to keep it (in case it was worth something) and just bought another set at the time so I had a complete set. Given that they said it was a common problem, I'm surprised that it seems unusual - and just a little bit glad that I hung on to it.
Any guesses on a value?